Beneficial and detrimental effects of choline chloride–oxalic acid deep eutectic solvent on biogas production
Graphical abstract
Introduction
Lignocellulosic biomass, which has a production rate of about 200 billion tons annually, is the most abundant natural raw material in the world (Kumar et al., 2015). Regarding simultaneous concerns with depletion and the environmental impact of fossil fuels, biomass has emerged as an alternative source of renewable energy, being one of the most appealing feedstocks for biorefinery processes (Surra et al., 2019). Aiming at a sustainable global environment, lignocellulosic biomass or its components (cellulose, hemicellulose, and lignin) can be fully converted into biofuels, energy sources, and chemicals (Tang et al., 2017). For decades, thermal and thermochemical technologies – combustion, pyrolysis, gasification – have been the main choice to process biomass. Nowadays, due to ecological concerns, biotransformation processes are of increasing interest. Anaerobic digestion (AD) is one of those biological processes, which allows the valorisation of organic wastes by producing biogas and digestate rich in nutrients (Surra et al., 2018). Moreover, it is one of the most economically reliable bioconversion technologies that has been implemented worldwide for the commercial production of electricity, heat, and bio-methane from organic wastes (Zheng et al., 2014). However, the utilization of lignocellulosic biomass as a source for biogas production via AD, has not been widely explored since lignocellulose feedstocks are highly recalcitrant to biochemical degradation due to their strongly bounded structure (Hou et al., 2017). Lignocellulosic materials are composed of different building blocks, mainly carbohydrates (cellulose and hemicellulose) and a small portion of non-carbohydrates (lignin, protein, and other extractives) (Geun et al., 2020). Highly crystalline cellulose makes up the core of lignocellulose, being bounded by a hemicellulose matrix and an outer lignin layer. The last one prevents the exposure of carbohydrates to both water and microorganisms’ activity (Abraham et al., 2020, Kumar et al., 2020). Therefore, for effective bioconversion of these carbohydrates, solvents are required to pre-treat and/or fractionate the desired fractions of lignocellulose. Several recalcitrant factors could be pointed out, but it is lignin that has been identified as the main responsible for inhibitory effects on the biological conversion of lignocellulosic biomass (Geun et al., 2020). Several lignin-targeting pre-treatments have been explored to reduce physical and chemical barriers and facilitate cellulose bio-accessibility. The pre-treatment step (which can comprise physical, chemical, physicochemical, and/or biological methods) can be carried out as a simple biomass dissolution step, or as a simultaneous pre-treatment and fractionation step. Lignin separation is also of interest for further valorisation, as it is a valuable source for aromatic platform chemicals (Alvarez-vasco et al., 2016, Chen et al., 2019, Wang et al., 2017).
Ionic liquids (ILs), which have intrinsic advantages such as low volatility, high thermal stability, and a high degree of intermolecular bonding, especially hydrogen bonding, when compared to common organic solvents, have been widely explored as chemical solvents in the pre-treatment of various types of biomass (Liu et al., 2012). Since the first studies, ILs have shown a high capacity for cellulose dissolution (Zhu et al., 2006), which may be advantageous, for instance, for bioethanol production (Liu et al., 2012, Zheng et al., 2014). The fractionation of lignin for further valorisation using ILs has also been a subject of study (Pu et al., 2007). However, such chemical treatments using ILs are generally performed under moderate to severe conditions, like high temperature (>100 °C) and long pre-treatment times (several days), which difficult the subsequent regeneration of lignin (Wang et al., 2017).
More recently, deep eutectic solvents (DES), another class of alternative solvents that are seen as ILs’ analogues, sharing several physicochemical properties, particularly the intricate hydrogen bonding network, have also been explored to overcome some of the ILs disadvantages, such as high cost and toxicity problems. DES were introduced and defined by Abbott (Abbott et al., 2003) as low melting temperature mixtures of two or more compounds, in which one is commonly a quaternary ammonium salt (Florindo et al., 2019), highlighting their interesting solvent properties. In fact, unlike ILs, DES are easily prepared and can be simply obtained by mixing two compounds, namely, a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA) in a certain molar ratio (Lima et al., 2018). Choline chloride (ChCl) is probably the most used HBA, frequently combined with carboxylic acids or glycols as HBD. Regarding biomass processing, ChCl has been undoubtedly widely explored, being combined with other inexpensive compounds such as oxalic acid, lactic acid, acetic acid, formic acid, ethylene glycol, glycerol, among others (Lynam et al., 2017, Xu et al., 2016). In general, DES have shown a high ability to solubilize lignin in opposition to cellulose and hemicellulose (Alvarez-vasco et al., 2016, Majová et al., 2017, Xu et al., 2016). That fact can be of great importance, since it may allow the total use of lignocellulosic biomass by the selective separation of lignin and, simultaneously, improve the biodegradability of holocellulose. In the last years, several works on delignification using DES have been published, approaching important aspects such as, for example, the yield of extractable lignin (van Osch et al., 2017), subsequent enzymatic hydrolysis of the residual holocellulose (Lynam et al., 2017), chemical state and purity of lignin after extraction (Alvarez-vasco et al., 2016), crystallinity index (Kumar et al., 2016), DES recycling (Majová et al., 2017), among others. However, despite these advances in the application of DES for lignocellulose pre-treatment, no research has been conducted to enhance methane production. Only Yu et al. (2019) have explored the use of ChCl as an additive in a liquid hot water pre-treatment of biomass which, by efficiently improving delignification, greatly improved bio-methane yield. However, after the pre-treatment step, the treated biomass was thoroughly washed with ethanol to remove ChCl and its soluble fractions, which precludes any conclusions about the direct effect of ChCl on methanogenic bacteria. DES toxicity to the most different living organisms has been little explored, not going beyond isolated studies with some specific bacteria (Bacillus subtilis, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa) (Hayyan et al., 2013, Wen et al., 2015), fish (Cyprinus carpio) (Juneidi et al., 2015), fungi (Aspergillus niger) (Juneidi et al., 2015), and plant (Allium sativum) and invertebrate (Hydra sinensis) (Wen et al., 2015).
Aiming to apply DES to bioprocessing, their toxicity to microbes must be determined, because, after a pre-treatment step, there will always be traces of DES in the residual holocellulose. In this work, an effective delignifying DES – ChCl:OA (1:1) (Majová et al., 2017, Mamilla et al., 2019) was selected and prepared to assess its possible toxicity and/or anaerobic biodegradability on methanogenic assays. The effect of ChCl:OA (1:1) DES concentration and of both DES components was monitored by biogas production. The final objective was to find a range of DES-content in which the biogas production was not negatively affected. This can be of great interest, for example, for the co-anaerobic digestion of lignocellulosic biomass, characterized by high C-content, with organic wastes characterized by high N-content (Surra et al., 2018).
Section snippets
DES preparation
For DES preparation, choline chloride (ChCl) and oxalic acid (OA) were purchased from Sigma-Aldrich, both with mass fraction purities ≥ 98%. ChCl and OA were combined in a 1:1 M ratio. This molar ratio was selected as is one of the most cited in the literature concerning the effectiveness of delignification of lignocellulosic biomass by ChCl:OA (Lee et al., 2021, Lee et al., 2019, Mamilla et al., 2019). The appropriate amount of each raw compound was placed in a sealed flask and heated up to
Physicochemical composition of anaerobic sludge and pre-hydrolysed OFMSW
Table 1 shows the physicochemical composition of the anaerobic sludge and pre-hydrolysed OFMSW used in the AD assays.
In this study, the composition of the pre-hydrolysed OFMSW showed a low variability during the study, which is represented in Table 1 by low standard deviations. This was due to the experimental strategy followed in this work, which was based on the use of a pre-hydrolysed OFMSW representative of the average concentration determined in the full-scale AD plant. As explained above,
Conclusions
The toxicity of ChCl:OA (1:1) DES on an anaerobic consortium was evaluated in the present work. At very low concentrations of ChCl:OA (1:1) DES (less than 0.8 g/L) no negative effect was registered. As the DES concentration increases up to 12.5 g/L, a lag-phase occurs and lasts longer as the DES concentration increases, which in itself is indicative of a certain inhibition degree. However, at the same time, the total production of biogas is improved. At concentrations higher than 19.8 g/L,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
F. Lima gratefully acknowledges the financial support of FCT/MCTES (Portugal) for the Ph.D. fellowship PD/BDE/114355/2016. This work was financed by the CQE project (UID/QUI/00100/2013), the Associated Laboratory for Green Chemistry LAQV-REQUIMTE (UID/QUI/50006/2013), and Solchemar Company.
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